As known in the art, images are provided to displays, such as computer or television (TV) displays, many times per second, i.e., at the display refresh or frame rate. Traditionally, displays do not gracefully support changes in the display refresh rate. During a rate change, they typically go blank and/or display corrupted images. As a result, such rate changes are performed relatively rarely, e.g., when changing the display mode, starting or stopping a full screen application (such as a video game) or when changing the power source between alternating current (AC) and direct current (DC).
It is also known that moving images appear best to the human eye when the display refresh rate matches the image update or frame rate (i.e., the rate at which the separate frames of an image are provided by the image source; also referred to hereinafter as the image rate) of the video or graphics (examples of what are referred to hereinafter as “content”). Depending on the source of the content, the image rate might typically be anywhere from 24 Hz to over 100 Hz, or 0 Hz in the case of a static image. Further still, certain types of content have varying frame rates, or frame rates may change as different content types are displayed, e.g., film material on a television typically has an image rate of 24 or 25 Hz, whereas video material may have images rates at 30, 50 or 60 Hz. Further still, displays are known to consume power based, in part, on how frequently they are refreshed; higher refresh rates consume more power, whereas lower rates consume less.
Various technologies are known that address issues similar to variable frame rates. For example, there are systems that employ “selective refresh”, such as the Digital Packet Video Link standard (DPVL) promulgated by the Video Electronics Standards Association (VESA). Based on comparisons with previous frames, selective refresh systems send only that portion of a display image that has changed each refresh period. Because the amount of data transmitted to the display from the image source (e.g., a graphics processor) can be less than full resolution, power savings may be realized. However, each display requires a full resolution frame buffer thereby adding to the cost of the display. Additionally, the display is still refreshed at a fixed rate independent of the image update rate.
Some high-end flight simulation systems are known to couple the refresh rate of the projector(s) to the three-dimensional (3D) graphics rendering rate (i.e., image rate) of the graphics processor(s) so that the image update rate dynamically adjusts to the image rate. However, these systems do not couple the display rate to the image rate for any other type of content, e.g., video image rate, nor are they configured to provide power savings. Furthermore, such systems are built using projection display systems, rather than displays more commonly associated with computers or televisions.
In both analog and digital TV systems, so-called “genlock” systems accommodate switches between sources possibly having slightly different refresh rates. In analog TV, this results in a period of corrupted images while the system adapts to the new frame rate. However, such systems cover only a fairly narrow range of vertical frequencies (related to the refresh rates) and no display refresh rate adjustment is done for power saving. Some digital TV systems avoid screen corruption when the video image rate changes through use of a complex frame rate conversion system. Even for digital TV systems that can smoothly adjust the display rate to match the input image rate, it is still expected that these systems (like analog TVs) can only work with a narrow range of vertical rates, and they do not take power savings into account.
Many digital TV systems (and computer video playback systems) use “inverse telecine” to convert the image rate of video back into progressive film rate images at 24 or 25 Hz. The converted video is then converted to the fixed refresh rate of the display. In these systems, the rate of the display is fixed and does not respond dynamically to changes in the rate of the video without causing some visible artifacts. Some systems will first do inverse telecine (if film content is detected) and then do an interpolated image rate conversion to the fixed display rate. These systems handle mixed film and video content well as the frame rate conversion can dynamically adjust the video image rate to the fixed rate of the display. However the results are still inferior to systems without these capabilities, and no power savings are taken into account.
More recently, Intel Corporation has proposed using a frame buffer in a display to allow the display interface between the image source and the display to be dynamically shut down when the display image is static, thereby saving power. However, the frame buffer in the display adds to the cost and power consumption of the display and, given that the display rate is fixed, no power is saved through control of the refresh rate. In another Intel proposal, the image source (e.g., the graphics processor) detects when the image is static and dynamically switches to a so-called “interlaced” display update whereby only half of the rows in the display are refreshed each period, resulting in power savings. However, this idea does not work with content comprising moving images, or at the least, will result in visual artifacts. Further still, this approach does not help with matching image rates of other content to the display refresh rate (or vice versa).
Therefore, it would be advantageous to provide techniques that allow for dynamically adjusting the refresh rate of the display. Displays with this technology can both lower power consumption when the image is changing slowly, and optimize the appearance of moving images by tuning the display frame rate to match the image frame rate.